For ascertaining mass flow of a medium, for example, a liquid and/or a gas, flowing in a pipeline, often, measuring devices are used, which are embodied as Coriolis mass flow meters and which, by means of a measuring transducer of the vibration type and operating and evaluating electronics attached thereto, induce Coriolis forces in the flowing medium and produce a measurement signal derived from these forces. Measuring devices of such type, utilizing a measuring transducer of the vibration type, have been known for a long time and have established themselves very well in industrial applications.
Known from the state of the art are various implementations of Coriolis mass flow meters having, in each case, a measuring transducer of the vibration type, wherein the measuring transducer comprises a straight measuring tube vibrating during operation and serving to convey the medium, and wherein the measuring tube communicates with the pipeline via an inlet side, inlet tube piece and an outlet side, outlet tube piece. Additionally, such implementations include: an exciter arrangement, which excites the measuring tube, during operation, by means of at least one electromechanical, especially electrodynamic, oscillation exciter acting thereon, to execute bending oscillations; and a sensor arrangement, which is equipped with oscillation pickups, especially electrodynamic oscillation pickups, for at least pointwise registering of inlet side and outlet side oscillations of the measuring tube and for producing electrical sensor signals influenced by the mass flow.
Straight measuring tubes, when excited to execute bending oscillations according to a first eigenoscillation form, the so-called driven, or, also, wanted, mode, effect Coriolis forces in the through flowing medium. The Coriolis forces lead, in turn, to bending oscillations coplanar with the excited bending oscillations according to a second eigenoscillation form of higher and/or lower order, the so-called Coriolis mode, superimposed on the excited bending oscillations, such that the oscillations registered on the inlet side and on the outlet side by means of the sensor arrangement exhibit a measurable phase difference dependent also on the mass flow.
Usually, the measuring tubes of measuring transducers of a such type, especially measuring transducers installed in Coriolis mass flow meters, are excited in the wanted mode to an instantaneous resonance frequency of the first eigenoscillation forms, especially at an oscillation amplitude controlled to be constant. Since this resonance frequency depends especially also on the instantaneous density of the medium, besides the mass flow, at least also the density of the flowing media can be directly measured by means of conventional Coriolis mass flow meters.
In order to achieve a mechanically balanced system, often two parallel measuring tubes are provided, which are excited to execute oscillations of opposite phase. Measuring transducers of the vibration type of such kind with two measuring tubes arranged in parallel are known, for example, from U.S. Pat. No. 4,768,385 or DE 34 43 234 A1.
Measuring transducers of the vibration type known from the state of the art often have a minimum tube diameter of 1 mm, when very small flows are to be measured. In many areas of application, such as, for example, biotechnology, chemistry or medical technology, however, it is often necessary to measure yet smaller mass flow, which, as a result, requires a yet smaller tube diameter of significantly less than 1 mm, in order to achieve a signal having an acceptable signal to noise ratio.